This invention relates to cellular mobile radiotelephone systems; and it relates, in particular, to antenna operating systems therefor.
Cellular mobile radiotelephone systems in which a predetermined set of frequency channels are reused in recurring patterns of adjacent cells are now known in the art. One example of such a system may be found in the A. E. Joel, Jr. U.S. Pat. No. 3,663,762. In the Joel patent, arrays of directional antennas are driven with a common signal and arranged to operate in an omnidirectional mode insofar as an individual cell is concerned. However, other cellular systems are known which employ directional antennas at selected points in a cell to achieve complete coverage of an individual cell by means of, for example, plural directional antennas.
It is also known in the art to direct an antenna for enhanced transmission between that antenna and another at a remote site as, for example, in the case of point-to-point radio transmission systems wherein a directional antenna is accurately aligned to direct a radio beam substantially only to a predetermined target site. Similarly, directional radio transmissions are used in radio location systems for motor vehicles as taught, for example, in the R. V. Tomkewitsch et al. U.S. Pat. No. 3,922,677 wherein it is noted that signals from above or below a predetermined directional antenna pattern are severely attenuated.
Further, in connection with cellular radio systems, they are often operated on a signal-to-interference (S/I) ratio basis using various techniques, including the frequency modulation capture effect, to contain interference appearing at one cell antenna site from nearby cochannel antenna sites. The perfect prevention of all such cochannel interference is usually too costly a goal for commercial systems so a goal of an average S/I level over the area of a cell, or a sector of a cell, and which is found to be acceptable to most users, is taken as a compromise solution. The term "average" is herein used in reference to S/I ratio in the common, i.e., not out of the ordinary, sense rather than in the arithmetic mean sense. That is, an average S/I ratio is one which is common to a large part of a region of interest. For example, in cellular radiotelephone systems a system operating objective is established so that among S/I measurements made throughout a region of interest no more than a predetermined small portion of the region will experience S/I below a predetermined minimum. For purposes of the present presentation, the minimum S/I for all but that small portion is said to be the average S/I. In one cellular system, a requirement that has been found to be useful is to have a minimum S/I of 17 db over at least 90% of a call coverage area such as a cell, or a predetermined sector of a cell; and such a criterion enables most users to consider transmission quality of a radio channel in that region to be good or excellent. Such an average S/I figure thus is not absolutely applicable for every call from any mobile unit at every point in the region involved.
It is desirable, both as a system design tool and for improving the S/I performance of a system or of operations in a particular region served by a system, to be able to improve signal-to-interference performance in as inexpensive a fashion as possible. One difficulty with such improvements that might be attempted by modifying an antenna operating system is that even for the smallest cells usually considered in the present state of the art there is more than an order of magnitude difference between the height of a cell site antenna from the ground on the one hand and the distance between the antenna and the edge of a cell region served thereby on the other hand. Therefore the portion of an antenna gain pattern which affects a remote edge of a serving cell region is substantially the same as the portion affecting a nearby cochannel interfering cell. Consequently, antennas in cellular systems have been typically oriented to direct their principal radiation along a plane which is essentially horizontal, i.e., parallel to a plane that is tangent to the average curvature of the earth at the base of the antenna site. Reliance has been placed upon propagation loss effects and the placement of cochannel antenna sites at a sufficient spacing to assure acceptably low levels of cochannel interference.